A communications network transports information or data among a number of locations. The information is usually presented to the network in the form of time-domain electrical signals and may represent any combination of telephony, video, or computer data in a variety of formats. To transport such information, a typical communications network consists of various physical sites (nodes) and information conduits (links) that interconnect the nodes. Each link carries information between nodes, while each node may contain equipment for combining, separating, transforming, conditioning, and routing the information signals.
Nodes are typically connected by many parallel links due to the need for capacity. While links in a network are currently being implemented by using electrical or microwave cables to carry RF, analog or digital signals, recent significant developments involving lasers, optical fibers, etc. in optical transmission elements enable network owners to replace or augment existing cable links. The main advantage of the optical transmission is extremely high modulation bandwidth of the optical carrier--orders of magnitude greater than with an electrical cable or microwave link. Thus, a present day optical carrier may be modulated with multi-gigabit-per-second data representing, for example, over 150,000 simultaneous telephone voice signals. Other significant advantages of the optical transmission are low attenuation, immunity to electrical noise, relatively good security of the transmitted signal and long distance transmission.
Despite the low attenuation of optical communications systems, an optical signal nevertheless requires amplification. The great distances spanned by fiber optic links often require the insertion of one or more optical amplifiers to strengthen the signal along the way. Fiber optic amplifiers are also needed before or after optical functions, such as filtering, for example, which may attenuate the signal even further.
Although many types of optical amplifiers are currently available, the most widely used is a fiber optic amplifier doped with rare earth elements, the most commonly used being dopant erbium, for example. When pumped at a suitable power and wavelength, the doped fiber amplifies an incoming data signal.
Operationally, a signal amplification optical fiber comprises a core portion to which a rare earth element has been added. An attenuated information carrying signal passing through such doped fiber is amplified as a result of the stimulated emission of the rare earth element. The rare earth element receives an energy emitted from a separate pump source for excitation--that is, the rare earth doped optical fiber is pumped at a pump wavelength of the rare earth dopant to cause population inversion of the dopant. This causes a signal propagating along the fiber at a signal wavelength to be amplified.
At the present time, two wavelength bands are prevalent in the transmission of signals in optical communications. Both bands are broadband, i.e., about 20-30 nm wide. One is simply known in the art as 1550 nm. This band is also referred to as the erbium band, because Er.sup.3+ is the dopant used for operation in this range. The other band is known as 1310 nm. At these 1550 and 1310 nm wavelength bands, the optical fibers can be optimized for low attenuation and dispersion characteristics.
It is worth noting that at the present time fiber optic amplifiers are separately designed and constructed for the 1550 band and the 1310 band. In particular, the current state of the art requires two pump sources for increasing signals in these two bands. This is because the erbium-doped amplifiers are not suited for amplifying optical signals in the 1310 band. Similarly, praseodymium doped (Pr.sup.3+) amplifiers, which amplify optical signals in the 1310 band, cannot boost optical signals having a wavelength within the 1550 nm range.
The present and foreseeable trend in the fiber optic communications suggests the use of multiple optical carriers utilizing a wavelength division multiplexing occupying both 1310 nm and 1550 nm bands along the same fiber link. A need, therefore, exists for an optical amplifier which uses a single pump source to provide an amplification in more than one band along the same fiber link.